Ok, I came here to show another teachers point of view. My mom has been an elementary school teacher for 25 years. I recently told her about this incident and here is her response.
In her opinion, calling the cops was a little over reactive. The only reason why my mom said it would be alright to call the cops would be if the child had a history of bad behavior or was a little "disturbed" in her words. She also said that if the child was a good child, with a history of good behavior, her school would take the child out of the cafeteria and take them to the office immediately to talk to the child about how bad it is to bring a knife to school, and ask how the knife got in her lunch box in the first place. In the words of another teacher, my own mother: the childs mother or father, whichever put the knife in the childs lunch box in the first place, is an idiot. Sorry if that was the wrong choice of word to use(idiot that is), but that's what my mom said.